To me, John Simm never really worked. Too manic.
What you really want is someone who can appear charming with a larger than life persona that can win the affections of the masses, but of course behind the scenes be sinister and evil. Generally older comes across better in the part, another reason why Simm didn't work for me. Having a beard too -it's a Master tradition, and maybe someone who has a foreign air about him, either in looks or speak.
A dark character with dark looks.
A younger picture here, sure -he's been crafting his technique since it was taken, but this fella looks like he'd fit the bill just perfect (watch out Clara):
http://3.bp.blogspot.com/-jffsSX8Stf...RolfHarris.jpg